Conventionally, for an insulated wire used in carrying out wiring of vehicle parts for an automobile and parts for an electrical/electronic appliance, an insulated wire is in widespread use in which a vinyl chloride resin composition prepared by adding a halogenous flame retardant thereto covers a conductor.
However, there is a problem that this kind of vinyl chloride resin composition contains halogen elements, so that it emits harmful halogenous gas into the atmosphere in case of car fire or at the time of disposing of the electrical/electronic appliance by incineration, causing environmental pollution.
Therefore, from the view point of reducing loads on the global environment, the vinyl chloride resin composition has been recently replaced with an olefin resin composition which contains an olefin resin such as polyethylene. The olefin resin is combustible, so that metal hydroxide such as magnesium hydroxide is added as a flame retardant to the olefin resin composition in order to secure sufficient flame retardancy.
When an insulated wire in which such an olefin resin composition covers a conductor is used in high temperature environment such as an automobile, heat resistance is required of the insulated wire. Hence, in order to improve heat resistance of the insulate wire, an insulating layer of the insulated wire is often subjected to a crosslinking treatment.
Examples of a method of the crosslinking treatment include an electron irradiation crosslinking method, a chemical crosslinking method and a water crosslinking method. Among them, the electron irradiation crosslinking method and the chemical crosslinking method need special crosslinking apparatuses which are high in cost and large in size, which leads to a problem of increased cost. Hence, the water crosslinking method which is free from such a problem and allows easy crosslinking has recently been in widespread use.
In addition, when an insulated wire in which such an olefin resin composition covers a conductor is used for interior wiring in an automobile or a building, flexibility is required of the insulated wire to facilitate the wiring. Especially in a case where metal hydroxide such as magnesium hydroxide is added as a flame retardant to the olefin resin composition, more flexibility is required of the insulated wire because a covering material of the insulated wire becomes hard.
In order to satisfy the requirements described above, Japanese Patent Application Unexamined Publication No. Hei 11-255977 discloses a flexible polyethylene resin composition which contains linear low-density polyethylene made by polymerization using a metallocene catalyst, and a silane crosslinking agent, and a flexible polyethylene resin composition which contains an ethylene copolymer such as an ethylene-ethylacrylate copolymer and an ethylene-vinyl acetate copolymer in addition to the above-described linear low-density polyethylene and silane crosslinking agent.